This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Heat assisted magnetic recording (HAMR) has been identified by the data storage industry as the technology for next generation data storage. As the density of data in a magnetic hard drive continues to increase and the relative bit size decreases, the magnetic storage medium must be made of a material with high coercivity to guarantee its stability. At some point as storage density increases, the bit size is so small and the coercivity correspondingly so high that the magnetic field used for writing data cannot be made strong enough, with the result that data can no longer be written to the hard drive medium using the magnetic field available in a read-write head. HAMR mitigates this problem by temporarily and locally changing the coercivity of the magnetic storage medium by using a laser beam to radiate the medium through an optical near field transducer (NFT) and raising the temperature of the medium above the Curie temperature. As a result, the medium temporarily loses coercivity and a realistically achievable magnetic field can be achieved which for the read-write head can write data to the recording medium.
Since the heating laser would have a wavelength of approximately 800 nm, the minimum spot size that can be produced using far-field optics would be on the order of 400 nm, determined by the physical diffraction limit. Such a spot size is too large as the next generation data storage require bit sizes of tens of nanometers, and thus conventional far-field optics is not suitable for HAMR and next generation magnetic data storage.
Nanoscale optical antennas or NFT are used to focus light to a nanoscale spot beyond the physical diffraction limit of light. FIG. 1a depicts a top view of an example of a planar nanoscale optical antenna similar to the one found in the prior art as provided in, e.g., U.S. Pat. No. 7,518,815 to Rottmayer et al., the difference being that the structure discussed in the '815 patent discloses a bowtie antenna, whereas FIG. 1a depicts a bowtie aperture antenna which has a reversed geometry. In the example shown in FIG. 1a, a nanoscale optical antenna, or NFT, 100 having a full bowtie shape aperture is shown. In the example shown, the antenna 100 comprises a thin metal film 101 having two wings (also referred to as aperture or tips) 102 in the thin metal film 101. The wings 102 which form the aperture exposes an optically transparent substrate 103 having the metal thin film 101 deposited thereon. A gap size 104 is depicted between the points of the aperture 102 and labeled “g.”
As discussed above, the planer bowtie-shaped aperture depicted in FIG. 1a is made in a thin metal film (about tens of nm to over 100 nm thick metal on a transparent substrate). The wings 102 of the antenna 100 are separated by the gap 104 of width g. When illuminated by a laser beam, electric potential and hence currents are induced in the wings 102 of the antenna 100, which flow to the tips 102. Because of the gap g (104), charges are accumulated at the tips 102, resulting in a displacement current across the gap which radiates similar to a Hertzian dipole. In other words, the antenna 100 receives radiation over a large area surrounding the aperture and re-radiates the received radiation through the displacement current formed in a small region in the gap g (104). The focusing resolution depends on the gap size g only, not the wavelength; and the transmission efficiency is orders of magnitude higher than a conventional aperture. FIG. 1b shows simulated results of the electric-field distribution at 1 nm above a bowtie aperture. An optical spot as small as 7 nm×4 nm is obtained, with electric field more than 240 times higher and the optical intensity 55,000 times higher than the incident laser intensity. The field produced by bowtie antenna diverges quickly within 10 s of nm, so it needs to be used in near field (as a near field transducer). In a magnetic disc drive, the read-write head is only a few nm above the storage medium during operation; therefore, the divergence of light from bowtie antenna is not an issue for the NFT in a magnetic disc drive.
The gap size 104 determines the size of a light spot transmitted when the antenna 100 is illuminated with a light source. In various embodiments the gap size 104 is desired to be in a range from about several nm to tens of nm. In other embodiments the gap size may be larger or smaller. Because the gap size 104 determines transmitted spot size, the designed and fabricated gap size 104 will be highly variable based on the desired application. In the example involving magnetic data recording wherein the antenna 100 is used to heat a spot in order to assist magnetic writing, the smaller spot size will allow a greater data density to be written to a magnetic storage medium. Thus, the gap size 104 of 5 nm and below is advantageous for HAMR.
Referring to FIG. 2 surrounding a bowtie aperture antenna, grooves at the entrance side (i.e., the side facing the incoming laser beam), can boost the field intensity by more than one order of magnitude. The mechanism of field enhancement is based on the grating effect, i.e., diffraction of propagating waves instead of other phenomena such as surface plasmons. Proper design of the grooves at the exit side can help to collimate the beam. This effect is due to the interference of the scattered surface plasmon polariton (SPP) waves at the edges of grooves that help to cancel or reduce the intensity of side lobes.
However, achieving such a gap size 104 is exceedingly difficult and costly. There are currently no cost-effective ways to generate gaps of such small size in a repeated high quality manner. Therefore, there is a need for new optical arrangements which can generate and utilize light spots tens of nanometers in size for writing data to the data storage devices.